Process for treatment of by-product gaseous mixture resulting from the reaction of methyl chloride with silicon

Abstract: The present invention is a process for the treatment of a by-product gaseous mixture resulting from the reaction of methyl chloride with silicon to form methylchlorosilanes. The process employs pressure swing adsorption to separate the by-product gases into a gaseous mixture enriched in methyl chloride and a gas reduced in methyl chloride content. The gaseous mixture enriched in methyl chloride content can be thermally oxidized. The gas reduced in methyl chloride content can be used as a fuel source or used as a feed to another process. By separating the methyl chloride from other combustible gases, it is possible to thermally oxidize greater quantities of methyl chloride, per unit time, for a given size thermal oxidizer.

Description: The present invention is a process for the treatment of a by-product gaseous mixture resulting from the reaction of methyl chloride with silicon to form methylchlorosilanes. The process employs pressure swing adsorption to separate the by-product gases into a gaseous mixture enriched in methyl chloride content and a gas reduced in methyl chloride content. The gaseous mixture enriched in methyl chloride content is thermally oxidized. The gaseous mixture reduced in methyl chloride content can be used as a fuel source or used as a feed to another process. An advantage of the present process is that, by separating the methyl chloride from other combustible gases, it is possible to thermally oxidize greater quantities of methyl chloride, per unit time, for a given size thermal oxidizer. The direct process for the production of methylchlorosilanes, as first described by Rochow, US-A 2,380,995 involves the reaction of methyl chloride with silicon in the presence of a suitable catalyst. A typical by-product of this process is a gaseous mixture comprising methyl chloride, methane, hydrogen, nitrogen and residual chlorosilanes. Federal and State environmental regulations require that by-product gases from chemical processes be controlled using environmentally sound methods. Typically, gases such as methane, hydrogen and nitrogen can be vented into the atmosphere without further treatment. However, chlorinated hydrocarbons, such as methyl chloride, are not considered environmentally safe and cannot be freely vented to the atmosphere. Thermal oxidation of the by-product gaseous mixture from the direct process is considered an environmentallysound method for disposing of methyl chloride and other gases. However, in practice, the methyl chloride may only comprise a minor component of the total combustible content of the by-product gaseous mixture. The capacity of a thermal oxidizer to thermally oxidize by-product gases is limited by the thermal load the thermal oxidizer is designed to handle. Therefore, by separating the methyl chloride from other combustible gases, it is possible to thermally oxidize greater quantities of methyl chloride, per unit time, for a given size thermal oxidizer. It is known that organic vapors can be recovered by passing the gas stream through an adsorbent bed containing a substance, such as activated carbon, to adsorb the vapors. It is also known to desorb the organic vapor from the bed by maintaining a steady temperature in the bed and lowering pressure within the bed. Known systems are described, for example, in US-A 3,225,518; US-A 4,056,369 and CZ-A 89396. Shen et al., Pollution Engineering, Oct. 1978, p. 46/47, even describes a process for the incineration of "silicone waste products, mainly chlorosilanes." CA-A-2069380 shows the adsorption of methyl chloride out of a gas containing methane. The cited documents do not recognize the advantages that can be achieved, when thermal oxidation is the selected method of disposing of methyl chloride, by partially or totally separating the methyl chloride from the other combustible gases and thermally oxidizing the gas enriched in methyl chloride separately. The present invention provides a process for treatment of the by-product gaseous mixture resulting from the reaction of methyl chloride with silicon to form organochlorosilanes. This by-product gaseous mixture typically comprises methyl chloride, methane, hydrogen, nitrogen and lessor amounts of chlorosilanes, ethane and other hydrocarbons. Our process uses pressure swing adsorption to separate the by-product gaseous mixture into a gaseous component enriched in the environmentally hazardous methyl chloride and a component reduced in methyl chloride content. Typically, the gas reduced in methyl chloride content is comprised of gases such as methane, hydrogen and nitrogen which can be used as a fuel source or as a feed to another process. The gaseous mixture enriched in methyl chloride content is then disposed of by thermal oxidation. The principal advantage of the present process is that it allows for more efficient use of a thermal oxidizer, by providing a feed gas enriched in methyl chloride and reduced in other gases. These latter gases would otherwise contribute to the design thermal load of the thermal oxidizer. Therefore, less capacity of the thermal oxidizer is used in the combustion of other combustible gases, leaving more capacity free for the thermal oxidation of methyl chloride. The process for treatment of a by-product gaseous mixture resulting from the reaction of silicon with methyl chloride to form methyl choride silanes of our invention comprises: (A) contacting the by-product gaseous mixture with an adsorbent capable of reversibly adsorbing the methyl chloride, to provide a gas reduced in methyl chloride, (B) lowering pressure of the by-product gaseous mixture in contact with the adsorbent to effect desorption of a gaseous mixture enriched in methyl chloride, and (C) thermally oxidizing the gaseous mixture enriched in methyl chloride. Figure 1 illustrates the configuration of an apparatus suitable for separating a by-product gaseous mixture into a gaseous mixture enriched in methyl chloride content and a gas reduced in methyl chloride content. Figure 2 illustrates the configuration of an apparatus suitable for the thermal oxidation of the gaseous mixture enriched in methyl chloride content. Contact of the gaseous mixture, resulting from the reaction of silicon with methyl chloride, with an adsorbent capable of reversibly adsorbing methyl chloride can be effected in any standard apparatus for contacting a gas with a solid under pressure. Figure 1 represents a useful configuration of such an apparatus and is provided by way of example. Referring to Figure 1, a by-product gaseous mixture is fed to pre-filter 1 . The purpose of pre-filter 1 is to remove hydrolyzable silanes present in the by-product gaseous mixture. Pre-filter 1 can be loaded with any material capable of removing hydrolyzable silanes from the by-product gaseous mixture, such as, activated carbon or hydrated silica gel. Pre-filter 1 is consider optional and the process can be run with or without pre-filter 1 . The pre-filtered by-product gaseous mixture is fed through flow regulators 7a through 7d to adsorption columns 2 through 5 . Flow regulators 7a through 7d are controlled by standard means to allow sequential operation of adsorption columns 2 through 5 . The number of adsorption columns employed in the process is not critical, however, it is preferred that at least two adsorption columns be employed, since this allows for continuous operation of our process. Generally, the number of columns used in the process is determined by the quantity of adsorbent contained in each column, the quantity of methyl chloride to be recovered, rate of by-product gas flow through the column and the adsorption/desorption rate of methyl chloride on the adsorbent. Standard means for sequencing flow of gases to columns can be used for the sequencing process. The column sequencing is effected such that, as the adsorption capacity of one column for methyl chloride is nearly saturated, the by-product gaseous mixture flow is switched to another adsorption column and desorption of the nearly saturated adsorption column is then effected. Adsorption columns 2 through 5 contain a packedbed of an adsorbent that is capable of reversibly adsorbing methyl chloride. The preferred adsorbent is silica gel. In a typical process, the by-product gaseous mixture is fed to adsorption columns 2 through 5 at an adsorption pressure sufficient to effect adsorption of methyl chloride. When the adsorbent is silica gel, a useful adsorption pressure is within a range of 0.8 to 3.1 MPa (125 to 450 psi) gauge pressure. A preferred adsorption pressure is within a range of 1.0 to 2.1 MPa (150 to 300 psi) gauge pressure. The optimal temperature of the by-product gaseous mixture fed to adsorption columns 2 through 5 is dependent upon the particular adsorbent used in the column. When the adsorbent is silica gel, it is preferred that the temperature be within a range of 10°C. to 300°C. Lower temperatures may be used, but at temperatures below 10°C., methane strongly adsorbs to silica gel and the separation of methyl chloride from methane is decreased. At temperatures above 300°C., the silica gel may deteriorate. A more preferred temperature is within a range of 24°C. to 65°C. The by-product gaseous mixture fed to columns 2 through 5 is separated into a gas component which passes through the column and is reduced in methyl chloride content. This gas component reduced in methyl chloride passes through one of regulators 8a through 8d , depending on which column is receiving the by-product gaseous mixture. The gas component reduced in methyl chloride can be used as a fuel source or used as a feed to another process. The by-product gaseous mixture fed to columns 2 through 5 is also separated into a component which adsorbs to the adsorbent. When the adsorbent is silica gel, the adsorbed component comprises methyl chloride, methane and minor levels of other hydrocarbons. Desorption of the methyl chloride and other adsorbed gases results in a gaseous mixture enriched in methyl chloride, when compared to the feed by-product gaseous mixture. Desorption of the adsorbent is effect by lowering the pressure within the adsorption column. When the adsorbent is silica gel, a useful desorption pressure is within a range of 0 to 206.8 kPa (0 to 30 psi) gauge pressure. Referring to Figure 1, by way of example, the desorption of adsorption column 2 is effected by first closing valves 7a and 8a . Valves 9a and 9c are then opened to partially pressurize column 4 . At the same time, valve 7b and 8b are opened to effect separation of by-product gas in column 3 . Valve 9c is then closed and valve 9d and 10d opened. A portion of the gas in column 2 is passed through column 5 to effect desorption of column 5 . Valves 9c , 9d and 10d are then closed. Valve 10a is then opened to vent any remaining pressure in column 2 to surge tank 6 . Next, valves 9b and 9a are opened. The gas which flows from column 3 through column 2 effects desorption of column 2 at a pressure within a range of 0 to 206.8 kPa (0 to 30 psi) gauge pressure. When the desorption is complete, valves 9a , 9b and 10a are closed. Valve 9c is opened to partially repressurize column 2 using gas from column 4 . Pressurization of column 2 back to adsorption pressure is completed by opening valve 8a and allowing part of the vent gas from column 5 to enter column 2 . The adsorption and desorption of methyl chloride in adsorption columns 3 through 5 are performed similarly by operation of the appropriate valves. Those skilled in the art will recognize that the valving can be controlled by standard instrumentation to effect sequential operations of columns 2 through 5 , allowing continuous operation of the process. Valve 11 and valve 12 are used to control the overall pressure of the system. The gaseous mixture enriched in methyl chloride, is passed from surge tank 6 to a thermal oxidizer. The thermal oxidizer can be any thermal oxidizer of standard design for the thermal oxidation of chlorinated hydrocarbons. Preferred is a thermal oxidation process similar to that illustrated in Figure 2. Referring to Figure 2, the gaseous mixture enriched in methyl chloride is fed from surge tank 6 to thermal oxidizer 13 . Thermal oxidizer 13 is maintained at a temperature within a range of 870°C. to 1200°C. A preferred temperature for thermal oxidizer 13 is within a range of 980°C. to 1100°C. The temperature of thermal oxidizer 13 is controlled within the desired temperature range by providing an auxiliary fuel source, for example natural gas, to raise the temperature of the thermal oxidizer and a steam or additional air source to lower the temperature of the thermal oxidizer, as required. Fan 14 provides air to thermal oxidizer 13 at a rate adequate to ensure at least a 25 percent stoichiometric excess of oxygen in the thermal oxidizer, in relation to combustible material feed to the thermal oxidizer. Exhaust gas from thermal oxidizer 13 is passed to quench chamber 15 , where the exhaust gas is cooled by use of water evaporation to between 80°C. and 85°C. If desired, a boiler may be included between thermal oxidizer 13 and quench chamber 15 to cool the flue gas and generate steam. The exhaust gas is than passed to HCl absorber 16 , for example, a countercurrent water scrubber or caustic scrubber. From HCl absorber 16 , the exhaust gas passes into dual chamber ionizing wet scrubber 17 of conventional design, where residual hydrogen chloride and particulates are removed from the exhaust gas. Other appropriate particulate scrubber means may be used in the process. The exhaust gas is drawn through induction fan 18 and is vented to the atmosphere through stack 19 . It is important that fan 18 maintain a negative pressure on the upstream system, since this minimizes corrosion problems typically associated with the incineration of chlorinated hydrocarbons. By "negative pressure" is meant a pressure less than atmospheric pressure.

Claims: A process for treatment of a by-product gaseous mixture resulting from the reaction of silicon with methyl chloride to form methylchlorosilanes, the by-product gaseous mixture comprising methyl chloride and methane, the process comprising: (A) contacting the by-product gaseous mixture with an adsorbent capable of reversibly adsorbing the methyl chloride, to provide a gas reduced in methyl chloride, (B) lowering pressure of the by-product gaseous mixture in contact with the adsorbent to effect desorption of a gaseous mixture enriched in methyl chloride, and (C) thermally oxidizing the gaseous mixture enriched in methyl chloride. A process according to claim 1 where the adsorbent is silica gel. A process according to claim 1 where the adsorption gauge pressure is within a range of 0.8 to 3.1 MPa (125 to 450 psi). A process according to claim 1 where the temperature is within a range of 24°C. to 65°C. A process according to claim 1 where thermally oxidizing the gaseous mixture enriched in methyl chloride is conducted in a thermal oxidizer at a temperature within a range of 980°C. to 1100°C. and exhaust gases from the thermal oxidizer are further treated by serially passing through a water quenching means, an HCl absorber means and an ionizing wet scrubber means, all maintained under a negative pressure. A process according to claim 1 where the process is a continuous process employing more than one bed of the adsorbent.

Inventors: HALM ROLAND LEE, OKESON KEVIN JAMES, WORDEN RICHARD GLEN, OKESON, KEVIN JAMES, WORDEN, RICHARD GLEN
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1994
Main classification: B01D53
IPC codes: B01D53, A62D3